Why Are Girls Diagnosed With Autism Spectrum Disorder Less Often Than Boys?

According to research, girls are diagnosed with autism spectrum disorder (ASD) less commonly than boys--a fact attributed to unique underlying genetic and neurobiological characteristics in girls and boys with ASD.

However, identifying behavioral differences between the sexes within ASD has been problematic as well as controversial.

Based on recent data, the prevalence of autism in girls is much lower than in boys. According to the CDC, while nearly 1 in 42 boys are diagnosed with autism in the U.S., the number is about 4 times lower in girls (1 in 189 girls).

Realizing this significant difference in prevalence, there is interest among scientists evaluating ASD in figuring out if behavioral and biological differences are leading to differences between boys and girls with autism.

While there are some indicators that there are differences in the genetic and neurobiological basis of autism in boys and girls, detection of behavioral differences has been somewhat inconsistent. The issue is that behavioral studies evaluating gender differences compare boys and girls with autism.

Christine Wu Nordahl, Ph.D, Assistant Professor, Department of Psychiatry and Behavioral Sciences, UC Davis School of Medicine, UC Davis MIND Institute decided to evaluate behavioral and social issues in a large group of children that included girls and boys with both autism and typical development. Her research team directly compared boys and girls with autism, but also sought to determine how boys and girls with autism compare in relation to their typically developing peers.

According to the data, the researchers found that behavioral differences between girls with autism and typically developing girls are much larger than differences between boys with autism and other typically developing boys.

The takeaway is that girls with autism have more severe social impairments than boys.

Nordahl’s research, “Sex Differences in Social Impairment in Preschool-Aged Children with Autism Spectrum Disorder," will be presented at the 2015 International Meeting for Autism Research (IMFAR), May 13-16 in Salt Lake City.

Although there have been ongoing studies to evaluate differences between boys and girls with autism--as a result of the higher prevalence of autism in boys--girls with autism are not studied as often.

“When girls and boys with autism are compared to sex-specific typically developing counterparts,” explained Nordahl, “the behavioral differences between girls with autism and typically developing girls are much larger than differences between boys with autism and typically developing boys.” “This suggests that girls can be considered more severely affected than boys, even if differences aren't obvious when girls and boys with autism are directly compared.”

Nordahl continued, “We don't yet know enough about females with autism because most research studies do not have equal numbers of females and males with autism in their samples. This is not surprising given that there are so many more males with autism than females.”

Nordahl explained that “we need to do a better job of trying to recruit females with autism into our studies so that we can fully explore differences between males and females with autism.”

Nordahl commented, “Our study adds to a growing body of evidence that there are differences between boys and girls with autism. These differences may affect how boys and girls are diagnosed with autism as well as potentially the types of treatments or interventions that boys and girls with autism receive. The ultimate goal is to understand the biological and behavioral differences between boys and girls with autism to try to understand the cause(s) of autism in boys and girls and to develop better treatments.”

“The male sex bias is one of the most distinctive features of autism spectrum disorder (ASD) and it has been observed consistently since 1943 when autism was first described by Kanner,” said Stephan Sanders, Ph.D.,Assistant Professor, UCSF School of Medicine. “The biological mechanism behind this sex bias remains unknown and is of great interest as a potential route to understand the etiology of ASD itself and possibly as an approach to developing therapeutics.”

Sanders explained that “there has been a lot of interest in the concept of a female protective effect under which affected females are predicted to have a higher burden of ASD risk factors.”

What the study shows, according to Sanders, is that “females with ASD have more severe features of ASD compared to typically developing females than the equivalent males.”

"This fits neatly with the genetic data on de novo mutations," explained Sanders, "that known ASD risk factors are observed more frequently in affected females than affected males, so that affected females are similarly more distinct from typically developing children than affected males."

Sanders believes that this raises important questions about girls with equivalent ASD characteristics to boys with ASD. "Namely, are they [females] being missed with conventional diagnostic outreach and testing? Do they have a recognized phenotype distinct from ASD?--and could they benefit from diagnosis and potentially behavioral interventions?”

I am an emergency physician on staff at Lenox Hill Hospital in New York City, where I have practiced for the past 15 years. I also serve as an adviser and editor to Medscape Emergency Medicine, an educational portal for physicians, and an affiliate of WebMD. My other time i...